A membrane electrode assembly used in a fuel cell is configured that a solid polymer electrolyte membrane is held between a hydrogen electrode and an air electrode, and the hydrogen electrode and the air electrode are each configured that reaction layers and diffusion layers are stacked in sequence from the solid polymer electrolyte membrane side.
A reaction layer comprises a mixture of a catalyst and an electrolyte and is required to have conductivity of electrons and protons and air permeability. Since the proton moves in the form of H3O+ along with water, the reaction layer is required to be maintained in a wet state. Of course, when an excess water content is present in the reaction layer, the air permeability is inhibited (so-called flooding phenomenon), and therefore, the water content of the reaction layer is required to be always maintained at a suitable amount.
In order to satisfy the above requirement, the present applicant has proposed a catalyst paste having a thin water film between a catalyst and an electrolyte in Patent Documents 1 and 2. In order to obtain such a catalyst paste, a pre-paste containing a catalyst previously mixed with water is provided and then mixed with an electrolyte solution, and a suitable stirring method is employed. In such a catalyst paste thus formed, a hydrophilic group of the electrolyte is drawn to a water film covering a catalyst to face the water film, and a hydrophilic region is formed between the electrolyte and the catalyst. The hydrophilic region becomes a water film. Hereinafter, a structure having a hydrophilic region between a catalyst and an electrolyte in a mixture of the catalyst and the electrolyte is also referred to as a “PFF structure”.
When the hydrophilic regions between the catalyst and the electrolyte are contiguous (namely, when the hydrophilic regions are not arranged in a spot-like pattern), uneven distribution of water in a reaction layer is prevented. Even when a fuel cell is operated in a low humidification environment, water is collectively present in the hydrophilic region, and therefore, overdrying can be prevented. In the operation in a high humidification environment, since excess water is discharged outside (toward a diffusion layer) through the hydrophilic region, flooding can be prevented.
See Patent Document 3 and Non-Patent Documents 1 to 3 as literature disclosing techniques related to the present invention.